GnuPG stands for GNU Privacy Guard and
is GNU's tool for secure communication and data storage. It can be
used to encrypt data and to create digital signatures. It includes
an advanced key management facility and is compliant with the
proposed OpenPGP Internet standard as described in RFC 2440.
As such, it is aimed to be compatible with PGP from NAI, Inc.

Yes. GnuPG is part of the GNU family of tools and applications built
and provided in accordance with the Free Software Foundation (FSF)
General Public License (GPL). Therefore the software is free to copy,
use, modify and distribute in accordance with that license. Please
read the file titled COPYING that accompanies the application for
more information.

Although GnuPG is being developed for several operating systems
(often in parallel), the conventions used in this FAQ reflect a
UNIX shell environment. For Win32 users, references to a shell
prompt (`$') should be interpreted as a command prompt (`>'),
directory names separated by a forward slash (`/') may need to be
converted to a back slash (`\'), and a tilde (`~') represents a
user's "home" directory (reference question 4.18 for an example).

Some command-lines presented in this FAQ are too long to properly
display in some browsers for the web page version of this file, and
have been split into two or more lines. For these commands please
remember to enter the entire command-string on one line or the
command will error, or at minimum not give the desired results.

Please keep in mind that this FAQ contains information that may not
apply to your particular version, as new features and bug fixes are
added on a continuing basis (reference the NEWS file included with
the source or package for noteworthy changes between versions). One
item to note is that starting with GnuPG version 1.1.92 the file
containing user options and settings has been renamed from "options"
to "gpg.conf". Information in the FAQ that relates to the options
file may be interchangable with the newer gpg.conf file in many
instances. See question 5.8 for details.

The documentation page is located at <http://www.gnupg.org/documentation/>.
Also, have a look at the HOWTOs and the GNU Privacy Handbook (GPH,
available in English, Spanish and Russian). The latter provides a
detailed user's guide to GnuPG. You'll also find a document about how
to convert from PGP 2.x to GnuPG.

PLEASE:
Before posting to a list, read this FAQ and the available documentation.
In addition, search the list archive - maybe your question has already
been discussed. This way you help people focus on topics that have not
yet been resolved.

The GnuPG source distribution contains a subdirectory:

./doc

where some additional documentation is located (mainly interesting
for hackers, not the casual user).

"Good" random numbers are crucial for the security of your encryption.
Different operating systems provide a variety of more or less quality
random data. Linux and *BSD provide kernel generated random data
through /dev/random - this should be the preferred choice on these
systems. Also Solaris users with the SUNWski package installed have
a /dev/random. In these cases, use the configure option:

--enable-static-rnd=linux

In addition, there's also the kernel random device by Andi Maier
<http://www.cosy.sbg.ac.at/~andi/SUNrand/>, but it's still beta. Use at your
own risk!

On other systems, the Entropy Gathering Daemon (EGD) is a good choice.
It is a perl-daemon that monitors system activity and hashes it into
random data. See the download page <http://www.gnupg.org/download/>
to obtain EGD. Use:

--enable-static-rnd=egd

here.

If the above options do not work, you can use the random number
generator "unix". This is very slow and should be avoided. The
random quality isn't very good so don't use it on sensitive data.

1024 bit for DSA signatures; even for plain ElGamal signatures.
This is sufficient as the size of the hash is probably the weakest
link if the key size is larger than 1024 bits. Encryption keys may
have greater sizes, but you should then check the fingerprint of
this key:

$ gpg --fingerprint <user ID>

As for the key algorithms, you should stick with the default (i.e.,
DSA signature and ElGamal encryption). An ElGamal signing key has
the following disadvantages: the signature is larger, it is hard
to create such a key useful for signatures which can withstand some
real world attacks, you don't get any extra security compared to
DSA, and there might be compatibility problems with certain PGP
versions. It has only been introduced because at the time it was
not clear whether there was a patent on DSA.

The problem here is that we need a lot of random bytes and for that
we (on Linux the /dev/random device) must collect some random data.
It is really not easy to fill the Linux internal entropy buffer; I
talked to Ted Ts'o and he commented that the best way to fill the
buffer is to play with your keyboard. Good security has its price.
What I do is to hit several times on the shift, control, alternate,
and caps lock keys, because these keys do not produce output to the
screen. This way you get your keys really fast (it's the same thing
PGP2 does).

Another problem might be another program which eats up your random
bytes (a program (look at your daemons) that reads from /dev/random).

Don't do this at all! You should never create keys or even use GnuPG
on a remote system because you normally have no physical control
over your secret key ring (which is in most cases vulnerable to
advanced dictionary attacks) - I strongly encourage everyone to only
create keys on a local computer (a disconnected laptop is probably
the best choice) and if you need it on your connected box (I know,
we all do this) be sure to have a strong password for both your
account and for your secret key, and that you can trust your system
administrator.

When I check GnuPG on a remote system via ssh (I have no Alpha here)
;-) I have the same problem. It takes a *very* long time to create
the keys, so I use a special option, --quick-random, to generate
insecure keys which are only good for some tests.

If you do a 'gpg --help', you will get two separate lists. The first
is a list of commands. The second is a list of options. Whenever you
run GPG, you must pick exactly one command (with one exception,
see below). You may pick one or more options. The command should,
just by convention, come at the end of the argument list, after all
the options. If the command takes a file (all the basic ones do),
the filename comes at the very end. So the basic way to run gpg is:

Some options take arguments. For example, the --output option (which
can be abbreviated as -o) is an option that takes a filename. The
option's argument must follow immediately after the option itself,
otherwise gpg doesn't know which option the argument is supposed to
paired with. As an option, --output and its filename must come before
the command. The --recipient (-r) option takes a name or keyID to
encrypt the message to, which must come right after the -r option.
The --encrypt (or -e) command comes after all the options and is
followed by the file you wish to encrypt. Therefore in this example
the command-line issued would be:

$ gpg -r alice -o secret.txt -e test.txt

If you write the options out in full, it is easier to read:

$ gpg --recipient alice --output secret.txt --encrypt test.txt

If you're encrypting to a file with the extension ".txt", then you'd
probably expect to see ASCII-armored text in the file (not binary),
so you need to add the --armor (-a) option, which doesn't take any
arguments:

If your filename begins with a hyphen (e.g. "-a.txt"), GnuPG assumes
this is an option and may complain. To avoid this you have to either
use "./-a.txt", or stop the option and command processing with two
hyphens: "-- -a.txt".

The exception to using only one command: signing and encrypting
at the same time. For this you can combine both commands, such as in:

$ gpg [--options] --sign --encrypt foo.txt

4.5) I can't delete a user ID on my secret keyring because it has
already been deleted on my public keyring. What can I do?

Because you can only select from the public key ring, there is no
direct way to do this. However it is not very complicated to do
anyway. Create a new user ID with exactly the same name and you
will see that there are now two identical user IDs on the secret
ring. Now select this user ID and delete it. Both user IDs will be
removed from the secret ring.

4.6) I can't delete my secret key because the public key disappeared.
What can I do?

To select a key a search is always done on the public keyring,
therefore it is not possible to select a secret key without
having the public key. Normally it should never happen that the
public key got lost but the secret key is still available. The
reality is different, so GnuPG implements a special way to deal
with it: Simply use the long keyID to specify the key to delete,
which can be obtained by using the --with-colons options (it is
the fifth field in the lines beginning with "sec").

If you've lost your public key and need to recreate it instead
for continued use with your secret key, you may be able to use
gpgsplit as detailed in question 4.21.

With GnuPG, the term "ownertrust" is used instead of "trust" to
help clarify that this is the value you have assigned to a key
to express how much you trust the owner of this key to correctly
sign (and thereby introduce) other keys. The "validity", or
calculated trust, is a value which indicates how much GnuPG
considers a key as being valid (that it really belongs to the
one who claims to be the owner of the key). For more information
on trust values see the chapter "The Web of Trust" in The GNU
Privacy Handbook.

Use "gpg --clearsign --not-dash-escaped ...". The problem with
--clearsign is that all lines starting with a dash are quoted with
"- "; obviously diff produces many lines starting with a dash and
these are then quoted and that is not good for a patch ;-). To use
a patch file without removing the cleartext signature, the special
option --not-dash-escaped may be used to suppress generation of
these escape sequences. You should not mail such a patch because
spaces and line endings are also subject to the signature and a
mailer may not preserve these. If you want to mail a file you can
simply sign it using your MUA (Mail User Agent).

4.10) How can I get rid of the Version and Comment headers in armored
messages?

This note is printed when UTF-8 mapping has to be done. Make sure
that the displayed character set is the one you have activated on
your system. Since "iso-8859-1" is the character set most used,
this is the default. You can change the charset with the option
"--charset". It is important that your active character set matches
the one displayed - if not, restrict yourself to plain 7 bit ASCII
and no mapping has to be done.

4.13) Why can't I decrypt files encrypted as symmetrical-only (-c) with
a version of GnuPG prior to 1.0.1.

There was a bug in GnuPG versions prior to 1.0.1 which affected files
only if 3DES or Twofish was used for symmetric-only encryption (this has
never been the default). The bug has been fixed, but to enable decryption
of old files you should run gpg with the option "--emulate-3des-s2k-bug",
decrypt the file and encrypt it again without this option.

NOTE: This option was removed in GnuPG development version 1.1.0 and later
updates, so you will need to use a version between 1.0.1 and 1.0.7 to
re-encrypt any affected files.

You should use the option --batch and don't use passphrases as
there is usually no way to store it more securely than on the
secret keyring itself. The suggested way to create keys for an
automated environment is:

On a secure machine:

If you want to do automatic signing, create a signing subkey
for your key (use the interactive key editing menu by issueing
the command 'gpg --edit-key keyID', enter "addkey" and select
the DSA key type).

Make sure that you use a passphrase (needed by the current
implementation).

gpg --export-secret-subkeys --no-comment foo >secring.auto

Copy secring.auto and the public keyring to a test directory.

Change to this directory.

gpg --homedir . --edit foo and use "passwd" to remove the
passphrase from the subkeys. You may also want to remove all
unused subkeys.

Copy secring.auto to a floppy and carry it to the target box.

On the target machine:

Install secring.auto as the secret keyring.

Now you can start your new service. It's also a good idea to
install an intrusion detection system so that you hopefully
get a notice of an successful intrusion, so that you in turn
can revoke all the subkeys installed on that machine and
install new subkeys.

Using GnuPG to encrypt email is one of the most popular uses.
Several mail clients or mail user agents (MUAs) support GnuPG to
varying degrees. Simplifying a bit, there are two ways mail can be
encrypted with GnuPG: the "old style" ASCII armor (i.e. cleartext
encryption), and RFC 2015 style (previously PGP/MIME, now OpenPGP).
The latter has full MIME support. Some MUAs support only one of
them, so whichever you actually use depends on your needs as well
as the capabilities of your addressee. As well, support may be
native to the MUA, or provided via "plug-ins" or external tools.

Users of Win32 MUAs that lack OpenPGP support may look into
using GPGrelay <http://gpgrelay.sourceforge.net>, a small
email-relaying server that uses GnuPG to enable many email clients
to send and receive emails that conform to PGP-MIME (RFC 2015).

This has been frequently requested. However, the current viewpoint
of the GnuPG maintainers is that this would lead to several security
issues and will therefore not be implemented in the foreseeable
future. However, for some areas of application gpgme could do the
trick. You'll find it at <ftp://ftp.gnupg.org/gcrypt/alpha/gpgme>.

4.17) I have successfully generated a revocation certificate, but I don't
understand how to send it to the key servers.

Most keyservers don't accept a 'bare' revocation certificate. You
have to import the certificate into gpg first:

GnuPG keeps several files in a special homedir directory. These
include the options file, pubring.gpg, secring.gpg, trustdb.gpg,
and others. GnuPG will always create and use these files. On unices,
the homedir is usually ~/.gnupg; on Windows "C:\gnupg\".

If you want to put your keyrings somewhere else, use the option:

--homedir /my/path/

to make GnuPG create all its files in that directory. Your keyring
will be "/my/path/pubring.gpg". This way you can store your secrets
on a floppy disk. Don't use "--keyring" as its purpose is to specify
additional keyring files.

Before you can verify the signature that accompanies a package,
you must first have the vendor, organisation, or issueing person's
key imported into your public keyring. To prevent GnuPG warning
messages the key should also be validated (or locally signed).

You will also need to download the detached signature file along
with the package. These files will usually have the same name as
the package, with either a binary (.sig) or ASCII armor (.asc)
extension.

Once their key has been imported, and the package and accompanying
signature files have been downloaded, use:

$ gpg --verify sigfile signed-file

If the signature file has the same base name as the package file,
the package can also be verified by specifying just the signature
file, as GnuPG will derive the package's file name from the name
given (less the .sig or .asc extension). For example, to verify a
package named foobar.tar.gz against its detached binary signature
file, use:

If you're wanting to create a keyring with only a subset of keys
selected from a master keyring (for a club, user group, or company
department for example), simply specify the keys you want to export:

$ gpg --armor --export key1 key2 key3 key4 > keys1-4.asc

4.21) I still have my secret key, but lost my public key. What can I do?

All OpenPGP secret keys have a copy of the public key inside them,
and in a worst-case scenario, you can create yourself a new public
key using the secret key.

A tool to convert a secret key into a public one has been included
(it's actually a new option for gpgsplit) and is available with GnuPG
versions 1.2.1 or later (or can be found in CVS). It works like this:

$ gpgsplit --no-split --secret-to-public secret.gpg >publickey.gpg

One should first try to export the secret key and convert just this
one. Using the entire secret keyring should work too. After this has
been done, the publickey.gpg file can be imported into GnuPG as usual.

4.22) Clearsigned messages sent from my web-mail account have an invalid
signature. Why?

Check to make sure the settings for your web-based email account
do not use HTML formatting for the pasted clearsigned message. This can
alter the message with embedded HTML markup tags or spaces, resulting
in an invalid signature. The recipient may be able to copy the signed
message block to a text file for verification, or the web email
service may allow you to attach the clearsigned message as a file
if plaintext messages are not an option.

PGP 2 uses the RSA and IDEA encryption algorithms. Whereas the RSA
patent has expired and RSA is included as of GnuPG 1.0.3, the IDEA
algorithm is still patented until 2007. Under certain conditions you
may use IDEA even today. In that case, you may refer to Question
3.3 about how to add IDEA support to GnuPG and read
<http://www.gnupg.org/gph/en/pgp2x.html> to perform the migration.

PGP, Inc. refuses to accept ElGamal keys of type 20 even for
encryption. They only support type 16 (which is identical at least
for decryption). To be more inter-operable, GnuPG (starting with
version 0.3.3) now also uses type 16 for the ElGamal subkey which is
created if the default key algorithm is chosen. You may add a type
16 ElGamal key to your public key, which is easy as your key
signatures are still valid.

PGP 5.x does not accept v4 signatures for data material but OpenPGP
requests generation of v4 signatures for all kind of data, that's why
GnuPG defaults to them. Use the option "--force-v3-sigs" to generate
v3 signatures for data.

Older PGPs probably bail out on some private comment packets used by
GnuPG. These packets are fully in compliance with OpenPGP; however
PGP is not really OpenPGP aware. A workaround is to export the
secret keys with this command:

$ gpg --export-secret-keys --no-comment -a your-KeyID

Another possibility is this: by default, GnuPG encrypts your secret
key using the Blowfish symmetric algorithm. Older PGPs will only
understand 3DES, CAST5, or IDEA symmetric algorithms. Using the
following method you can re-encrypt your secret gpg key with a
different algo:

No. The ~/.gnupg/options file has been renamed to ~/.gnupg/gpg.conf for
new installs as of version 1.1.92. If an existing ~/.gnupg/options file
is found during an upgrade it will still be used, but this change was
required to have a more consistent naming scheme with forthcoming tools.
An existing options file can be renamed to gpg.conf for users upgrading,
or receiving the message that the "old default options file" is ignored
(occurs if both a gpg.conf and an options file are found).

PGP can (for most key types) use secret keys generated by GnuPG. The
problems that come up occasionally are generally because GnuPG
supports a few more features from the OpenPGP standard than PGP does.
If your secret key has any of those features in use, then PGP will
reject the key or you will have problems communicating later. Note
that PGP doesn't do ElGamal signing keys at all, so they are not
usable with any version.

These instructions should work for GnuPG 1.0.7 and later, and PGP
7.0.3 and later.

Start by editing the key. Most of this line is not really necessary
as the default values are correct, but it does not hurt to repeat the
values, as this will override them in case you have something else set
in your options file.

Turn off some features. Set the list of preferred ciphers, hashes,
and compression algorithms to things that PGP can handle. (Yes, I
know this is an odd list of ciphers, but this is what PGP itself uses,
minus IDEA).

> setpref S9 S8 S7 S3 S2 S10 H2 H3 Z1 Z0

Now put the list of preferences onto the key.

> updpref

Finally we must decrypt and re-encrypt the key, making sure that we
encrypt with a cipher that PGP likes. We set this up in the --edit
line above, so now we just need to change the passphrase to make it
take effect. You can use the same passphrase if you like, or take
this opportunity to actually change it.

On many systems this program should be installed as setuid(root).
This is necessary to lock memory pages. Locking memory pages prevents
the operating system from writing them to disk and thereby keeping your
secret keys really secret. If you get no warning message about insecure
memory your operating system supports locking without being root. The
program drops root privileges as soon as locked memory is allocated.

To setuid(root) permissions on the gpg binary you can either use:

$ chmod u+s /path/to/gpg

or

$ chmod 4755 /path/to/gpg

Some refrain from using setuid(root) unless absolutely required for
security reasons. Please check with your system administrator if you
are not able to make these determinations yourself.

On UnixWare 2.x and 7.x you should install GnuPG with the 'plock'
privilege to get the same effect:

$ filepriv -f plock /path/to/gpg

If you can't or don't want to install GnuPG setuid(root), you can
use the option "--no-secmem-warning" or put:

no-secmem-warning

in your ~/.gnupg/options or ~/.gnupg/gpg.conf file (this disables
the warning).

On some systems (e.g., Windows) GnuPG does not lock memory pages
and older GnuPG versions (<=1.0.4) issue the warning:

gpg: Please note that you don't have secure memory

This warning can't be switched off by the above option because it
was thought to be too serious an issue. However, it confused users
too much, so the warning was eventually removed.

LFS works correctly in post-1.0.4 versions. If configure doesn't
detect it, try a different (i.e., better) compiler. egcs 1.1.2 works
fine, other gccs sometimes don't. BTW, several compilation problems
of GnuPG 1.0.3 and 1.0.4 on HP-UX and Solaris were due to broken LFS
support.

6.3) In the edit menu the trust values are not displayed correctly after
signing uids. Why?

This happens because some information is stored immediately in
the trustdb, but the actual trust calculation can be done after the
save command. This is a "not easy to fix" design bug which will be
addressed in some future release.

6.6) An ElGamal signature does not verify anymore since version 1.0.2 ...

6.8) When I use --clearsign, the plain text has sometimes extra dashes
in it - why?

This is called dash-escaped text and is required by OpenPGP.
It always happens when a line starts with a dash ("-") and is
needed to make the lines that structure signature and text
(i.e., "-----BEGIN PGP SIGNATURE-----") to be the only lines
that start with two dashes.

If you use GnuPG to process those messages, the extra dashes
are removed. Good mail clients remove those extra dashes when
displaying such a message.

Due to different message formats GnuPG is not always able to split
a file with multiple signatures unambiguously into its parts. This
error message informs you that there is something wrong with the input.

The only way to have multiple signatures in a file is by using the
OpenPGP format with one-pass-signature packets (which is GnuPG's
default) or the cleartext signed format.

As of 1.0.3, keys generated with gpg are created with preferences to
TWOFISH (and AES since 1.0.4) and that also means that they have the
capability to use the new MDC encryption method. This will go into
OpenPGP soon, and is also suppoted by PGP 7. This new method avoids
a (not so new) attack on all email encryption systems.

This in turn means that pre-1.0.3 gpg binaries have problems with
newer keys. Because of security and bug fixes, you should keep your
GnuPG installation in a recent state anyway. As a workaround, you can
force gpg to use a previous default cipher algo by putting:

If you just generated a new key and get this message while
encrypting, you've witnessed a bug in 1.0.4. It uses the new AES
cipher Rijndael that is incorrectly being referred as "deprecated".
Ignore this warning, more recent versions of gpg are corrected.

Due to constraints in most libc implementations, dates beyond
2038-01-19 can't be displayed correctly. 64-bit OSes are not
affected by this problem. To avoid printing wrong dates, GnuPG
instead prints some question marks. To see the correct value, you
can use the options --with-colons and --fixed-list-mode.

Are you sure that it's not been mentioned somewhere on the mailing
lists? Did you have a look at the bug list (you'll find a link to
the list of reported bugs on the documentation page). If you're not
sure about it being a bug, you can send mail to the gnupg-devel
list. Otherwise, use the GUUG bug tracking system
<http://bugs.guug.de/Reporting.html>.

According to OpenPGP, GnuPG encodes user ID strings (and other
things) using UTF-8. In this encoding of Unicode, most national
characters get encoded as two- or three-byte sequences. For
example, &aring; (0xE5 in ISO-8859-1) becomes &Atilde;&yen; (0xC3,
0xA5). This might also be the reason why keyservers can't find
your key.

6.20) I upgraded to GnuPG version 1.0.7 and now it takes longer to load my
keyrings. What can I do?

The way signature states are stored has changed so that v3 signatures
can be supported. You can use the new --rebuild-keydb-caches migration
command, which was built into this release and increases the speed of
many operations for existing keyrings.

6.21) Doesn't a fully trusted user ID on a key prevent warning messages
when encrypting to other IDs on the key?

No. That was actually a key validity bug in GnuPG 1.2.1 and earlier
versions. As part of the development of GnuPG 1.2.2, a bug was
discovered in the key validation code. This bug causes keys with
more than one user ID to give all user IDs on the key the amount of
validity given to the most-valid key. The bug has been fixed in GnuPG
release 1.2.2, and upgrading is the recommended fix for this problem.
More information and a patch for a some pre-1.2.2 versions of GnuPG
can be found at:

6.22) I just compiled GnuPG from source on my GNU/Linux RPM-based system
and it's not working. Why?

Many GNU/Linux distributions that are RPM-based will install a
version of GnuPG as part of its standard installation, placing the
binaries in the /usr/bin directory. Later, compiling and installing
GnuPG from source other than from a source RPM won't normally
overwrite these files, as the default location for placement of
GnuPG binaries is in /usr/local/bin unless the '--prefix' switch
is used during compile to specify an alternate location. Since the
/usr/bin directory more than likely appears in your path before
/usr/local/bin, the older RPM-version binaries will continue to
be used when called since they were not replaced.

To resolve this, uninstall the RPM-based version with 'rpm -e gnupg'
before installing the binaries compiled from source. If dependency
errors are displayed when attempting to uninstall the RPM (such as
when Red Hat's up2date is also installed, which uses GnuPG), uninstall
the RPM with 'rpm -e gnupg --nodeps' to force the uninstall. Any
dependent files should be automatically replaced during the install
of the compiled version. If the default /usr/local/bin directory is
used, some packages such as SuSE's Yast Online Update may need to be
configured to look for GnuPG binaries in the /usr/local/bin directory,
or symlinks can be created in /usr/bin that point to the binaries
located in /usr/local/bin.

Data that is encrypted with a public key can only be decrypted by
the matching secret key. The secret key is protected by a password,
the public key is not.

So to send your friend a message, you would encrypt your message
with his public key, and he would only be able to decrypt it by
having the secret key and putting in the password to use his secret
key.

GnuPG is also useful for signing things. Files that are encrypted
with the secret key can be decrypted with the public key. To sign
something, a hash is taken of the data, and then the hash is in some
form encoded with the secret key. If someone has your public key, they
can verify that it is from you and that it hasn't changed by checking
the encoded form of the hash with the public key.

A keyring is just a large file that stores keys. You have a public
keyring where you store yours and your friend's public keys. You have
a secret keyring that you keep your secret key on, and should be very
careful with. Never ever give anyone else access to it and use a *good*
passphrase to protect the data in it.

You can 'conventionally' encrypt something by using the option 'gpg -c'.
It is encrypted using a passphrase, and does not use public and secret
keys. If the person you send the data to knows that passphrase, they
can decrypt it. This is usually most useful for encrypting things to
yourself, although you can encrypt things to your own public key in the
same way. It should be used for communication with partners you know
and where it is easy to exchange the passphrases (e.g. with your boy
friend or your wife). The advantage is that you can change the
passphrase from time to time and decrease the risk, that many old
messages may be decrypted by people who accidently got your passphrase.

You can add and copy keys to and from your keyring with the 'gpg
--import' and 'gpg --export' option. 'gpg --export-secret-keys' will
export secret keys. This is normally not useful, but you can generate
the key on one machine then move it to another machine.

Keys can be signed under the 'gpg --edit-key' option. When you sign a
key, you are saying that you are certain that the key belongs to the
person it says it comes from. You should be very sure that is really
that person: You should verify the key fingerprint with:

$ gpg --fingerprint KeyID

over the phone (if you really know the voice of the other person), at
a key signing party (which are often held at computer conferences),
or at a meeting of your local GNU/Linux User Group.

Hmm, what else. You may use the option '-o filename' to force output
to this filename (use '-' to force output to stdout). '-r' just lets
you specify the recipient (which public key you encrypt with) on the
command line instead of typing it interactively.

Oh yeah, this is important. By default all data is encrypted in some
weird binary format. If you want to have things appear in ASCII text
that is readable, just add the '-a' option. But the preferred method
is to use a MIME aware mail reader (Mutt, Pine and many more).

There is a small security glitch in the OpenPGP (and therefore GnuPG)
system; to avoid this you should always sign and encrypt a message
instead of only encrypting it.

These are ElGamal keys generated by GnuPG in v3 (RFC 1991) packets.
The OpenPGP draft later changed the algorithm identifier for ElGamal
keys which are usable for signatures and encryption from 16 to 20.
GnuPG now uses 20 when it generates new ElGamal keys but still
accepts 16 (which is according to OpenPGP "encryption only") if this
key is in a v3 packet. GnuPG is the only program which had used
these v3 ElGamal keys - so this assumption is quite safe.

It works more or less like PGP. The difference is that the trust is
computed at the time it is needed. This is one of the reasons for
the trustdb which holds a list of valid key signatures. If you are
not running in batch mode you will be asked to assign a trust
parameter (ownertrust) to a key.

You can see the validity (calculated trust value) using this
command.

$ gpg --list-keys --with-colons

If the first field is "pub" or "uid", the second field shows you the
trust:

o = Unknown (this key is new to the system)
e = The key has expired
q = Undefined (no value assigned)
n = Don't trust this key at all
m = There is marginal trust in this key
f = The key is full trusted
u = The key is ultimately trusted; this is only used
for keys for which the secret key is also available.
r = The key has been revoked
d = The key has been disabled

The value in the "pub" record is the best one of all "uid" records.
You can get a list of the assigned trust values (how much you trust
the owner to correctly sign another person's key) with:

$ gpg --list-ownertrust

The first field is the fingerprint of the primary key, the second
field is the assigned value:

- = No ownertrust value yet assigned or calculated.
n = Never trust this keyholder to correctly verify others signatures.
m = Have marginal trust in the keyholders capability to sign other
keys.
f = Assume that the key holder really knows how to sign keys.
u = No need to trust ourself because we have the secret key.

Keep these values confidential because they express your opinions
about others. PGP stores this information with the keyring thus it
is not a good idea to publish a PGP keyring instead of exporting the
keyring. GnuPG stores the trust in the trustdb.gpg file so it is okay
to give a gpg keyring away (but we have a --export command too).

This is the internal representation of a user ID in the trustdb.
"C26EE891" is the keyid, "298" is the local ID (a record number in
the trustdb) and "09FB" is the last two bytes of a ripe-md-160 hash
of the user ID for this key.

While checking the validity of a key, GnuPG sometimes prints some
information which is prefixed with information about the checked
item.

"key 12345678.3456"

This is about the key with key ID 12345678 and the internal number
3456, which is the record number of the so called directory record
in the trustdb.

"uid 12345678.3456/ACDE"

This is about the user ID for the same key. To identify the user ID
the last two bytes of a ripe-md-160 over the user ID ring is printed.

"sig 12345678.3456/ACDE/9A8B7C6D"

This is about the signature with key ID 9A8B7C6D for the above key
and user ID, if it is a signature which is direct on a key, the user
ID part is empty (..//..).

7.6) Are the header lines of a cleartext signature part of the signed
material?

No. For example you can add or remove "Comment:" lines. They have
a purpose like the mail header lines. However a "Hash:" line is
needed for OpenPGP signatures to tell the parser which hash
algorithm to use.

The list of preferred algorithms is a list of cipher, hash and
compression algorithms stored in the self-signature of a key during
key generation. When you encrypt a document, GnuPG uses this list
(which is then part of a public key) to determine which algorithms
to use. Basically it tells other people what algorithms the
recipient is able to handle and provides an order of preference.

In version 1.0.7 or later, you can use the edit menu and set the
new list of preference using the command "setpref"; the format of
this command resembles the output of the command "pref". The
preference is not changed immediately but the set preference will
be used when a new user ID is created. If you want to update the
preferences for existing user IDs, select those user IDs (or select
none to update all) and enter the command "updpref". Note that the
timestamp of the self-signature is increased by one second when
running this command.

Many thanks to Nils Ellmenreich for maintaining this FAQ file for
such a long time, Werner Koch for the original FAQ file, and to all
posters to gnupg-users and gnupg-devel. They all provided most of
the answers.

Also thanks to Casper Dik for providing us with a script to generate
this FAQ (he uses it for the excellent Solaris2 FAQ).